Passivation for a semiconductor light emitting device

ABSTRACT

In embodiments of the invention, a passivation layer is disposed over a side of a semiconductor structure including a light emitting layer disposed between an n-type region and a p-type region. A material configured to adhere to an underfill is disposed over an etched surface of the semiconductor structure.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a divisional of U.S. application Ser. No. 13/904,299, filed May29, 2013, and titled “Passivation for a Semiconductor Light EmittingDevice”, which is a continuation of U.S. application Ser. No.12/795,272, filed Jun. 7, 2010, and titled “Passivation for aSemiconductor Light Emitting Device”, issued as U.S. Pat. No. 8,471,282on Jun. 25, 2013. U.S. application Ser. Nos. 13/904,299 and 12/795,272are incorporated herein by reference.

BACKGROUND

1. Field of Invention

The present invention relates to a passivation layer on a semiconductorlight emitting device.

2. Description of Related Art

Semiconductor light-emitting devices including light emitting diodes(LEDs), resonant cavity light emitting diodes (RCLEDs), vertical cavitylaser diodes (VCSELs), and edge emitting lasers are among the mostefficient light sources currently available. Materials systems currentlyof interest in the manufacture of high-brightness light emitting devicescapable of operation across the visible spectrum include Group III-Vsemiconductors, particularly binary, ternary, and quaternary alloys ofgallium, aluminum, indium, and nitrogen, also referred to as III-nitridematerials. Typically, III-nitride light emitting devices are fabricatedby epitaxially growing a stack of semiconductor layers of differentcompositions and dopant concentrations on a sapphire, silicon carbide,III-nitride, or other suitable substrate by metal-organic chemical vapordeposition (MOCVD), molecular beam epitaxy (MBE), or other epitaxialtechniques. The stack often includes one or more n-type layers dopedwith, for example, Si, formed over the substrate, one or more lightemitting layers in an active region formed over the n-type layer orlayers, and one or more p-type layers doped with, for example, Mg,formed over the active region. Electrical contacts are formed on the n-and p-type regions.

U.S. 2006/0281203 describes “techniques for mounting LEDs for packagingand for removing the growth substrate of the LEDs.” An underfillmaterial is injected between the carrier and die to provide support tothe epitaxial structure during substrate removal. A portion of thesemiconductor structure is supported by the interconnects between thedie and the carrier, and a portion is supported by the underfill. Toprevent cracking when the semiconductor structure is exposed to stress,for example during substrate removal, the mechanical compliance andcoefficient of thermal expansion of the interconnects and the underfillare preferably matched. Examples of suitable underfill materials includeFB4511 epoxy available from Henkel Corporation, and silicones and otherepoxies loaded with inorganic materials such as silica or alumina toreach the desired coefficient of thermal expansion and mechanicalcompliance. Since the underfill provides support for the epitaxiallayers, it is desirable for the underfill to fill all gaps between theinterconnects and to avoid the formation of air bubbles which mayencourage cracking of the epitaxial structure during substrate removal.Accordingly, the surface tension of the underfill material may beselected such that the underfill fills all gaps between theinterconnects. Alternatively, a partial vacuum may be created on a sideof the gap between the carrier and the die opposite the side where theunderfill is injected, to draw the underfill into all gaps between theinterconnects.

SUMMARY

It is an object of the invention to provide a passivation layer for asemiconductor light emitting device.

In a method according to some embodiments of the invention, a structureis provided. The structure includes a wafer comprising a plurality ofsemiconductor light emitting devices, each light emitting devicecomprising a light emitting layer disposed between an n-type region anda p-type region. The structure further includes a passivation layerdisposed on a side of at least one of the semiconductor light emittingdevices and a first material disposed on the wafer between twosemiconductor light emitting devices. The method further includesdisposing a second material between the structure and a mount. The firstmaterial is configured to adhere to the second material. The structureis attached to the mount.

In some embodiments of the invention, a device includes a semiconductorstructure comprising a light emitting layer disposed between an n-typeregion and a p-type region. A passivation layer is disposed over atleast part of a sidewall of the semiconductor structure. A materialconfigured to adhere to an underfill is disposed over an etched surfaceof the semiconductor structure.

In some embodiments of the invention, a structure includes a wafercomprising a plurality of semiconductor light emitting devices, eachlight emitting device comprising a light emitting layer disposed betweenan n-type region and a p-type region. A passivation layer is disposed ona side of at least one of the semiconductor light emitting devices. Amaterial configured to adhere to an underfill is disposed on the waferbetween two semiconductor light emitting devices.

The passivation layer may be, in some embodiments, the underfill, adielectric layer, or a multilayer stack. The passivation layer may beconfigured to prevent contaminants from contacting the semiconductorlight emitting device, which may improve the performance of a device andavoid device failure. The material disposed on the wafer between twosemiconductor light emitting devices improves the adherence of theunderfill to the wafer, which may also improve passivation of thedevice.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a portion of a wafer of flip chip light emittingdevices with a dielectric layer disposed in a street between adjacentdevices (adjacent devices are not shown).

FIG. 2 illustrates a portion of a wafer of flip chip light emittingdevices with a dielectric layer disposed over a bonding metal and in astreet between adjacent devices (adjacent devices are not shown).

FIG. 3 illustrates portion of a wafer of flip chip light emittingdevices with a reflective dielectric stack disposed in a street betweenadjacent devices (adjacent devices are not shown).

DETAILED DESCRIPTION

Though in the examples below the semiconductor light emitting device isa III-nitride LED that emits blue or UV light, semiconductor devicesbesides LEDs such as laser diodes and semiconductor devices made fromother materials systems such as other III-V materials, III-phosphide,III-arsenide, II-VI materials, or Si-based materials may be used.

FIG. 1 illustrates a portion of a wafer of thin film flip chip lightemitting devices. To form the structure illustrated in FIG. 1, first asemiconductor structure 20 is grown over a growth substrate. Thesemiconductor structure 20 includes a light emitting or active region 24sandwiched between n- and p-type regions 22 and 26. An n-type region 22is typically grown first over the growth substrate and may includemultiple layers of different compositions and dopant concentrationincluding, for example, preparation layers such as buffer layers ornucleation layers, which may be n-type or not intentionally doped, andn- or even p-type device layers designed for particular optical orelectrical properties desirable for the light emitting region toefficiently emit light. A light emitting or active region 24 is grownover the n-type region 22. Examples of suitable light emitting regionsinclude a single thick or thin light emitting layer, or a multiplequantum well light emitting region including multiple thin or thicklight emitting layers separated by barrier layers. A p-type region 26 isgrown over the light emitting region 24. Like the n-type region 22, thep-type region 26 may include multiple layers of different composition,thickness, and dopant concentration, including layers that are notintentionally doped, or n-type layers.

One or more p-contact metals 28, such as, for example, silver, isdeposited on the p-type region 26, then portions of the p-type regionand active region are etched away to expose a portion 35 of an n-typelayer on which an n-contact 40 is later formed. The p-contact 28 may besealed by one or more guard layers 30 and 32 disposed beside and overp-contact 28. Guard layers 30 and 32 may be, for example, a dielectriclayer with openings that expose p-contact 28 or, as illustrated in FIG.1, one or more metal layers such as TiW. One or more dielectric layers34, such as, for example, Si₃N₄ formed by, for example, plasma enhancedchemical vapor deposition (PECVD) is disposed over guard layer 30 toelectrically isolate the conductive structure connected to the p-typeregion (including p-contact 28 and guard layers 30 and 32) and then-contact 40. Openings are formed in dielectric layer 34 in the regions35 where n-contact 40 electrically contacts n-type region 22, then then-contact metal 40, such as, for example, aluminum, is formed. A bondingmetal 42, such as, for example, gold, is formed over n-contact 40 tofacilitate bonding each device to a mount 56 and electrical contact withthe mount.

The p-contact 28 and n-contact 40 are formed on the same side of thesemiconductor structure. In some embodiments either or both then-contact 40 and the p-contact 28 are reflective and the device ismounted such that light is extracted through the top of the device inthe orientation illustrated in FIG. 1. In some embodiments, the contactsmay be limited in extent or made transparent, and the device may bemounted such that light is extracted through the surface on which thecontacts are formed.

The wafer of devices is attached to a mount 56, for example byultrasonic bonding, thermosonic bonding, or thermocompression bonding ofbonding layer 42 to a bonding layer (not shown in FIG. 1) formed onmount 56. Before, during, or after attaching wafer 10 to mount 56, anunderfill 58 is disposed in any areas between the semiconductor devicewafer 10 and mount 56 where the semiconductor device wafer 10 is notsupported by the bonding layers. The underfill 58 may be, for example, adielectric layer or other solid layer that is deposited on wafer 10 ormount 56 and patterned, or a material such as silicone, epoxy, or anyother suitable material that is injected between wafer 10 and mount 56after bonding then cured into a solid. Underfill 58 supports wafer 10during processing and operation of the semiconductor devices, inparticular during removal of the growth substrate. For example, asapphire substrate may be removed by shining a laser through thesubstrate to melt a layer of semiconductor material in direct contactwith the substrate, a process that may create stress in thesemiconductor structure. In some embodiments, the semiconductor layerexposed by removing the growth substrate is patterned or roughened,which may improve light extraction from the device. In some embodiments,the growth substrate may remain part of the device.

As illustrated in FIG. 1, in an area 36 between two devices, known as a“street,” a portion 38 of dielectric layer 34 described above is left.As illustrated in FIG. 1, the portion 38 of dielectric layer 34 in thestreet is disposed over an etched surface of the semiconductor structure20; i.e., the surface of n-type region 22 exposed by etching a mesa. Thestreet may be, for example, between 1 and 10 microns wide and is oftenabout five microns wide. The street is cut, for example by sawing, bylaser scribing, or by laser scribing and breaking, to singulate thewafer into individual dice. The wafer may be diced, for example, after awafer of devices is attached to a wafer of mounts, or before attachingto the mount. Dielectric portion 38 may improve adhesion of underfill 58to wafer 10, particularly at the interface between semiconductormaterial (n-type region 22) and underfill 58. Improving the adhesion ofthe underfill to the wafer may improve the reliability of thesemiconductor devices by forming a seal that prevents contaminants fromreaching the device, and may reduce damage or avoid failure caused byinsufficient support of wafer 10.

FIG. 2 illustrates a portion of a wafer of thin film flip chip lightemitting devices. The structure illustrated in FIG. 2 includes apassivation layer 44. Semiconductor structure 20 is grown, thenp-contact 28, guard layers 30 and 32, dielectric 34, n-contact 40, andbonding layer 42 are formed and patterned as described above inreference to FIG. 1. Passivation layer 44 is deposited over bondinglayer 42. Passivation layer 44 may be any suitable material such as, forexample, an insulating or dielectric layer, AlN, TiN, SiO₂,SiN_(x)O_(y), SiN_(x), or Si₃N₄, formed by, for example, any suitablemethod including sputtering, e-beam evaporation, CVD, PECVD, orspin-coating or dip-coating the structure with appropriate precursormaterials, then curing the precursor materials to form a high densityinsulating dielectric. Passivation layer 44 may be a single layer or amulti-layer structure. Passivation layer 44 is patterned by conventionalphotolithography techniques to form one or more openings 46 which allowthe structure to be connected to a mount (not shown in FIG. 2) by, forexample, attaching bonding layer 42 to a bonding layer formed on amount, as described above in reference to FIG. 1. Passivation layer 44may be left in streets 36 between adjacent devices.

Passivation layer 44 covers the device, except in areas where conductivepaths are required for attaching to electrodes on the mount. Passivationlayer 44 seals the side of the device by coating the side of bondinglayer 42 and n-contact 40. In the areas where it is formed, passivationlayer 44 passivates the structure by protecting the device fromcorrosion, etching, oxidation, and other processes that may damage thedevice during operation or processing. For example, passivation layer 44may reduce or prevent the intrusion of corrosive species such as watervapor, which may improve the performance of the device and/or reducefailure rates. In some embodiments, the thickness of passivation layer44 is selected to reflect any light emitted by active region 24 that maybe incident on passivation layer 44. Passivation layer 44 may improvethe adhesion of an underfill to the wafer, as described above inreference to FIG. 1.

In FIG. 3, the passivation layer 44 of FIG. 2 is replaced by amultilayer dielectric stack 48. Multilayer dielectric stack 48 mayinclude several pairs of alternating layers of two materials 50 and 52with different refractive indices. Examples of suitable materialsinclude, for example, SiN_(x), Si₃N₄ and SiO₂ formed by, for example,the techniques described above for forming passivation layer 44 of FIG.2, or any other suitable technique. Materials 50 and 52 are selected toform a stack 48 that reflects any light from active region 24 that isincident on stack 48. Openings 54 are formed in stack 48 where necessaryfor electrical and/or physical contact to bonding layer 42. Multilayerstack 48 may improve the adhesion of an underfill to the wafer, asdescribed above in reference to FIG. 1.

In some embodiments, as illustrated in FIG. 3, in or adjacent to street36, all or a portion 35 of the semiconductor structure remaining afterthe mesa etch may be removed to expose the growth substrate (not shownin FIG. 3). The passivation layer (underfill 58 in FIG. 1, passivationlayer 44 in FIG. 2, and stack 48 in FIG. 3) may extend over the side ofthe semiconductor structure where the entire thickness of thesemiconductor structure 20 is removed, as illustrated in FIG. 3.Extending the passivation layer over the edge of semiconductor structure20 may further improve passivation of the structure and extraction fromthe device, by reflecting light incident on the interface between n-typeregion 22 and stack 48.

Having described the invention in detail, those skilled in the art willappreciate that, given the present disclosure, modifications may be madeto the invention without departing from the spirit of the inventiveconcept described herein. Therefore, it is not intended that the scopeof the invention be limited to the specific embodiments illustrated anddescribed.

What is being claimed is:
 1. A method comprising: growing asemiconductor structure comprising a light emitting layer disposedbetween an n-type region and a p-type region; disposing an underfillbeneath the semiconductor structure; and disposing a passivation layerover at least part of a sidewall of the semiconductor structure, whereinthe passivation layer is configured to adhere to the underfill and issandwiched between the semiconductor structure and the underfill;wherein at the sidewall the underfill and the passivation layer form aseal that prevents contaminants from reaching the semiconductorstructure.
 2. The method of claim 1 further comprising attaching thesemiconductor structure to a mount, wherein the underfill is disposedbetween the semiconductor structure and the mount.
 3. The method ofclaim 1 wherein the passivation layer is selected from a groupconsisting of an insulating layer, a dielectric layer, AlN, TiN, SiO₂,SiN_(x)O_(y), SiN_(x), and Si₃N₄.
 4. The method of claim 1 wherein thepassivation layer is configured to reflect light emitted by the lightemitting layer.
 5. The method of claim 1 wherein the passivation layeris a multilayer dielectric stack.
 6. A method comprising: forming awafer comprising a plurality of semiconductor light emitting devices,each light emitting device comprising a light emitting layer disposedbetween an n-type region and a p-type region; disposing a passivationlayer on a side of at least one of the semiconductor light emittingdevices and on the wafer between two semiconductor light emittingdevices, wherein the passivation layer is configured to adhere to anunderfill and is sandwiched between the semiconductor light emittingdevice and the underfill, and cutting the wafer to singulate the twosemiconductor light emitting devices in an area where the underfill andthe passivation layer form a seal that prevents contaminants fromreaching the semiconductor structure.
 7. The method of claim 6 furthercomprising attaching the wafer to a mount, wherein the underfill isdisposed between the semiconductor structure and the mount.
 8. Themethod of claim 6 wherein the passivation layer is selected from a groupconsisting of an insulating layer, a dielectric layer, AlN, TiN, SiO₂,SiN_(x)O_(y), SiN_(x), and Si₃N₄.
 9. The method of claim 6 wherein thepassivation layer is configured to reflect light emitted by the lightemitting layer.